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Stool Osmotic Gap

Stool  osmotic gap =
    Stool osmolality - 2 x (stool Na + stool K)

Normal fecal fluid values:
  Osmolality: ~290 mOsm/kg   Na+:  ~30 mmol/L   K+: ~75 mmol/L
Stool Na:
Stool K:
Stool Osm:


Secretory diarrhea

Diarrhea associated with a stool osmolal gap less than 50 mOsm/kg.

Potential causes:
  -Bile salt enteropathy
  -Congenital diarrhea
  -Drugs: (e.g. colchicine, SSRIs, prostaglandins, quinine, 
       cholinesterase inhibitors).
  -Enterotoxin-producing bacterial pathogens:( e.g. clostridium,
       cholera and shigella.)
  -Laxative abuse
  -Lymphocytic or collagenous colitis
  -Neuroendocrine tumors (Vipomas, gastrinomas, carcinoid
  -Rectal villous adenoma

Osmotic diarrhea

Diarrhea associated with a stool osmolal gap greater than 100 mOsm/kg.

Potential causes:
-Hexitols (poorly absorbed): sorbitol, mannitol, xylitol).
-Sugar intolerance (e.g. lactose intolerance).
-Unabsorbable water-soluble solutes used as laxatives: (PEG, Mg salts, Na Phosphate).


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